Just to prove it was not a fluke I have repeated the TBB speed tests on Chrome and then FF and on a sigle download FF gets about 20mbps and Chrome see about 29mbps so it would seem that There is a problem with FF at least on Linux and the TBB speed test (the one using Flash). There is still a difference between the single and 6x tests on Chrome, 29mbps and 37mbps respectively.
While using the HG635 TT want me to turn off the router for 30 minutes and then power it back up to see if it makes any difference, dont see why this might make a difference but hey I'll give it a go...
Stuart